A typical technology is known by which a communication device synchronizes the time thereof with the time of a time server by means of communication with the time server. A protocol specification IEEE 1588-2008 (hereinafter, referred to as IEEE 1588) is disclosed for highly accurate synchronization of the time within, for example, a Local Area Network (LAN). In this technology, a communication device refers to the time announced via multicast by a time server compliant to IEEE 1588 and refers to a communication delay occurring between the communication device and the time server, and accordingly synchronizes the time thereof with the time of the time server. Meanwhile, when a plurality of time servers is present in the same network in a communicable manner, only the time server having the highest priority announces the time thereof. Moreover, also disclosed is a method by which a plurality of time servers announces the respective time. That method is called Alternate master option. According to that method, a communication device periodically receives time from the time servers equipped with the Alterna master option function. Thus, even if any of the time servers is malfunctioning, the communication device is able to receive the timings from the other time servers. That enables the communication device to maintain the timing accuracy.
However, in IEEE 1588, all time servers in the same domain determine the respective internal conditions by means of negotiation, and make changes in the operation of communicating messages according to the internal conditions. Herein, the behavior of any one time server affects the other time servers. For example, in case a particular time server starts malfunctioning and sends an Announce message in which the parameter indicating the priority of that time server is set to a value not intended by the administrator, then the other time servers may temporarily stop announcing the respective time. Besides, at the time of restoring a time server or while performing maintenance, if incorrect settings are performed with respect to a time server to be newly connected to the network, then the other time servers may temporarily stop announcing the respective time. As a result, the communication device suffers from a timing drift during the period of not receiving the timings from the time servers.
Further, a time synchronization system is disclosed in which a single time server is connected to each of two networks and a communication device is connected to both the networks so as to make the time servers redundant. Here, Grandmaster represents the time servers and Ordinary clock represents the communication device. Of the two time servers each connected to a network, one time server serves as a main time server and the other time server serves as a sub-time server. The communication device synchronizes with the time of the main time server and, when communication cannot be performed with the main time server, switches to the sub-time server as the target time server for time synchronization. In this way, by making the communication paths and the time servers redundant, the reliability of the time synchronization system is enhanced.
However, in the technology disclosed above, when a plurality of time servers is installed in the same network and is made redundant, there is a risk that the time accuracy of the communication device may not be maintained in case a time server is malfunctioning or is being restored.